1. Field of the Invention
The present invention relates to a semiconductor device sealed with a resin, that is, a plastic package, and more particularly to a plastic package of which electrodes of a semiconductor chip and lead terminals are electrically connected to each other by a metal plate such as copper. The present application is based on Japanese Patent Application No. 264083/2000 and Japanese Patent Application No. 013869/2001, the disclosures of which are incorporated herein by reference.
2. Description of the Related Art
A semiconductor device sealed with a resin, that is, a plastic package, comprises a semiconductor chip (also called a pellet or a die), wiring materials forming outer terminals such as a lead frame, and a molding resin for sealing the semiconductor chip bonded to the wiring materials and inner leads.
Examples of the wiring materials are a tape carrier (a film carrier), a printed circuit plate, and the like other than a lead frame. As the molding resin, an epoxy resin is mainly used.
A wire bonding method has conventionally been employed in many cases as a method for electrically connecting electrodes of a semiconductor chip and lead terminals of wiring materials. FIGS. 1A and 1B are illustrations showing one example of a conventional semiconductor device 1 employing wire bonding method. FIG. 1A is a plane view and FIG. 1B is a cross-section cut along the line I-I′ of FIG. 1A.
As shown in FIGS. 1A and 1B, the semiconductor device 1 is an 8-pin SOP (Small Outline Package) produced by mounting and bonding a semiconductor chip 10 comprising a MOSFET on and with a lead frame 20, forming an electrical connection by a bonding wire 7, and sealing with a molding resin 8. The semiconductor chip 10 comprises a gate electrode 11 and three source electrodes 12 on the upper face and a drain electrode (not shown in the illustrations) on the bottom face. The lead frame 20 is provided with leads projected from opposite sides in the package; four drain leads 21 on the left side of FIG. 1A and one gate lead 22 and three source leads 23 on the right side of FIG. 1A. The drain leads 21 are integrally formed in the inside of the package and due to that, an island part 24 is formed. The gate lead 22 has an inner lead terminal part 25 (hereinafter referred to as a gate terminal) in the inside of the package. Each of three source leads 23 has an inner lead terminal part 26 (hereinafter referred to as a source terminal) in the inside of the package. The semiconductor chip 10 is bonded to the island part 24 through a die bond material 9 and the drain electrode (not shown in the illustrations) is electrically connected with the island part 24. The gate electrode 11 on the upper face of the semiconductor chip 10 and the gate terminal 25 of the lead frame 20 are connected with each other by the bonding wire 7. Also, the source electrodes 12 and the source terminals 26 of the lead frame 20 are connected with each other by the bonding wire 7. The semiconductor chip 10, inner leads (including the island part 24, the gate terminal 25, and the source terminals 26), and the bonding wire 7 are sealed with a molding resin 8 to form a package.
The above described semiconductor device 1 is an example of a power transistor for high electric current use and in order to lower the resistance, source electrodes 12 and the gate terminal 25 are connected with the bonding wire 7 of a gold wire or the like as much as possible.
However, by the wire bonding method using a costly thin metal wire such as a gold wire or the like, problems are caused. Not only is the fabrication cost considerably increased but also disconnection takes place in the thin metal wire to make the package unsuitable for high electric current use. Recently, a method for forming an electrical connection using a metal plate such as copper has been proposed (Japanese Laid-Open Patent Application Heisei 8-148623) in a power transistor for high electric current use. As compared with a metal wire, the metal plate has advantages such as decrease of the resistance and improvement of heat radiation since the cross-sectional surface area can be widened.
However, in a plastic package in which electrodes of a semiconductor chip and the lead terminals of wiring materials are electrically connected by a metal plate, there exist the following problems.
At first, description will be given regarding an application example of a plastic package in which electrodes of a semiconductor chip and lead terminals of wiring materials are electrically connected with a metal plate. FIGS. 2A and 2B are illustrations of a semiconductor device 2 using a metal plate for connection. FIG. 2A is a plane view and FIG. 2B is a cross-section cut along the line II-II′ in FIG. 2A.
As shown in FIGS. 2A and 2B, the semiconductor device 2 is an 8-pin SOP produced by mounting and bonding a semiconductor chip 30 comprising a MOSFET on and with a lead frame 40, forming an electrical connection by a bonding wire 7 and a metal plate 50 of such as a copper plate, and sealing by a molding resin 8. The semiconductor chip 30 comprises a gate electrode 31 and a single source electrode 32 with a large surface area on the upper face and a drain electrode (not shown in the illustrations) on the bottom face. The lead frame 40 is provided with leads projected from opposite sides of the package; four drain leads 41 on the left side of FIG. 2A and one gate lead 42 and three source leads 43 on the right side of FIG. 2A. The drain leads 41 are integrally formed in the inside of the package and due to that, an island part 44 is formed. The gate lead 42 has an inner lead terminal part 45 in the inside of the package. The three source leads 43 are integrally formed in the inside of the package and have a single wide width source terminal 46. The semiconductor chip 30 is bonded to the island part 44 through a die bond material 9 and the drain electrode (not shown in the illustrations) is electrically connected with the island part 44. The gate electrode 31 on the upper face of the semiconductor chip 30 and the gate terminal 45 of the lead frame 40 are connected with each other by the bonding wire 7. Also, the source electrode 32 and the source terminal 46 of the lead frame 40 are connected with each other by the metal plate 50 of copper. The metal plate 50 is bonded to the source electrode 32 in one end and to the source terminal 46 in the other end by a conductive paste 6 to electrically connect the source electrode 32 and the source terminal 46. The semiconductor chip 30, inner leads (including the island part 44, the gate terminal 45, and the source terminal 46), the bonding wire 7, and the metal plate 50 are sealed with a molding resin 8 to form a package.
As shown in FIG. 2A, the metal plate 50 is formed to have about a half width of the external size of the package and the cross-section surface area is extremely widened as compared with that of a gold wire. Consequently, the resistance of the package is lowered.
A semiconductor device is to be exposed to severe environments in which the temperature, the humidity, and the pressure are fluctuated at the time of being mounted and used after the packaging. Because the semiconductor device is repeatedly subjected to temperature alteration, the bonding of the metal plate and the molding resin ocassionally becomes broken, causing separation of the metal plate from the resin. Then, water and a corrosive gas may penetrate the semiconductor chip through the parted interfaces to cause corrosion of the semiconductor chip. As a result, the reliability of the semiconductor device is lowered.
In the case of using a metal wiring, even if such separation from the molding resin occurs, since the cross-sectional surface area of a gold wire is small, the penetration route of the water and a gas is relatively narrow and therefore, water and the gas scarcely penetrate the semiconductor chip through the interface between the metal wire and the molding resin. This mitigates the extent that the reliability of the semiconductor device is deteriorated.
However, since the cross-sectional surface area of the metal plate is large as compared with that of the metal wire and further the cross-sectional surface area is intentionally widened based on the requirement of decrease of the resistance and improvement of heat radiation, the contact surface area with the molding resin is naturally widened and the penetration route of water and the gas becomes wide. This adversely affects the reliability of the semiconductor device, as it is deteriorated by the water and gas penetration.
Further, although in some cases, a solder paste and a resin type conductive paste are used for bonding the lead terminals and the metal plate, the solder paste and the conductive paste sometimes flow out of the bonding range of the metal plate and the lead terminals. In the case where the solder paste and the conductive paste flow from the lead terminals in the extended direction of the leads and are spread from the lead terminals to the package outer circumference position and to the peripheral part of the package outer circumference, the conductive paste and the like decrease the adhesion strength of the molding resin and cause the separation of the molding resin. As a result, water and the gas may easily penetrate the semiconductor chip from the outside and cause corrosion of the semiconductor chip, resulting in reduced reliability of the semiconductor device.
On the other hand, it is desired to mount a metal plate on electrodes and lead terminals of the semiconductor chip at a high precision.
The present invention has been developed while taking the above described conventional techniques into consideration. Regarding a semiconductor device (a plastic package) in which electrodes of a semiconductor chip and lead terminals are electrically connected with a metal plate, such as copper, and which is sealed with a resin, the present invention improves the sealing property of the molding resin and improves the reliability of the semiconductor device by improving the adhesion property between the metal plate and the molding resin.
Further, the present invention improves the adhesion property between the leads and the sealing resin and sealing property of the molding resin and improves the reliability of the semiconductor device by preventing a conductive bonding material employed for bonding the lead terminals and the metal plate from being spread to the outside of the bonding range between the metal plate and the lead terminals, particularly to the outside of the package on the leads.
Furthermore, the present invention provides a semiconductor device in which a metal plate is easily and highly precisely mounted on electrodes of a semiconductor chip and the lead terminals of a lead frame.